The present invention relates to photolithography methods and, more particularly, to optical lithography methods for forming fine-sized patterns on a wafer or other substrates, such as using a photolithographic mask and a projection lens.
In existing projection systems used in optical photolithography, a quasi-monochromatic, spatially incoherent light source of wavelength λ is used to illuminate a photolithographic mask having various patterns, such as a periodic pattern of equally spaced lines. The illuminating beam is usually collimated to ensure a highly uniform intensity distribution at the plane of the mask, and an adjustable condenser stop is used to control the degree of coherence of the illuminating beam. The light is transmitted through the mask and collected by a projection lens which images the mask patterns onto a wafer located at the image projection plane, typically at a predetermined reduction ratio.
In such projection systems, a lines and spaces pattern on the mask diffracts the illuminating beam and forms a plurality of light beams that pass through a projection lens. An optical image of the lines and spaces pattern is formed on the wafer when the light beams interfere with each other. The smaller the pitch of the lines and spaces pattern on the mask, however, the larger the angle at which light diffracted by the mask spreads. Thus, if the pitch of the lines and spaces pattern is sufficiently small, the angle defined by two adjacent diffracted light beams is large enough for the first order and higher order diffracted light beams to impinge outside the projection lens so that no optical image is formed on the wafer.
To print such smaller lines and spaces patterns on a wafer, projection lenses having larger numerical apertures are used to accept larger incidence angles of diffracted light. The numerical aperture (NA) of a projection lens is defined as NA=sin θ, where θ is the half-angle of a cone that is subtended by the clear aperture of the projection lens at the wafer. As an alternative, the exposure wavelength is decreased to decrease the angle of diffraction occurring at the mask. In both methods, however, as the lines and spaces patterns that are to be printed approach submicron sizes, the contrast of the patterns formed on the wafer deteriorates, and the depth of focus decreases. As a result, neither alternative is practical at these smaller dimensions.
As an example, dynamic random access memory devices (DRAMs) typically include a semiconductor memory cell array formed of a plurality of memory cells arranged in rows and columns and include a plurality of bit lines as well as a plurality of word lines that intersect the bit lines. Each memory cell of the array is located at the intersection of a respective word line and a respective bit line and includes a capacitor for storing data and a transistor for switching, such as a planar or vertical MOS transistor. The word line is connected to the gate of the switching transistor, and the bit line is connected to the source or drain of the switching transistor. When the transistor of the memory cell is switched on by a signal on the word line, a data signal is transferred from the capacitor of the memory cell to the bit line connected to the memory cell or from the bit line connected to the memory cell to the capacitor of the memory cell.
Current DRAM technology often uses a buried capacitor DRAM memory in which memory bits are constructed in pairs to allow sharing of a bit line contact. The sharing of the bit line contact significantly reduces the overall cell size. Typically, the memory bit pair includes an active area (AA), a pair of word lines, a bit line contact, a metal or polysilicon bit line, and a pair of cell capacitors.
The bit line pitch, i.e., the width of the bit line plus the distance between adjacent bit lines, typically determines the active area pitch and the capacitor pitch. The active area width is typically adjusted to maximize the transistor drive and minimize the transistor-to-transistor leakage.
The word line pitch typically determines the space available for the bit line contact, the transistor length, the active area space, and the capacitor length. Each of these dimensions must be optimized to maximize device capacitance, minimize device leakage and maximize process yield.
As semiconductor devices become increasingly smaller, the active area pitch, the bit line pitch, and the word line pitch decrease accordingly. The segmented line mask patterns that define the features likewise are smaller and become increasingly difficult to print because of line shortening effects. Moreover, the active area, bit line and word line mask patterns not only include lines and spaces, which are used in the array areas of these levels, but also include small isolated spaces, which likewise are difficult to print because of the limited size of the open areas that can be printed using the masks, as well as larger sized features used in the support circuitry regions.
To form such finer lines and spaces patterns, a phase-shifting mask is used. The optical phase of light transmitted through some or all of the mask is changed by changing the thickness of various regions of the mask, either by depositing additional transparent material where needed or by removing a thin layer from the mask at specific locations, thereby selectively adjusting the transmitted optical phase at these locations. The phase-shifting mask diffracts the light transmitted by the mask pattern and causes it to interfere destructively or constructively based on the location on the mask pattern, thereby increasing the depth of focus and allowing for the printing of finer lines and spaces patterns.
Typically, an attenuated phase-shift mask (PSM) is used to print the active area pattern as well as other patterns. The dark areas of the mask typically transmit the incident light at a reduced intensity but with a 180 degrees phase shift relative to the clear areas of the mask so that the light transmitted by the dark areas interfere with the light transmitted by the clear areas. The use of attenuated phase-sift masks, however, requires off-axis illumination (OAI) and thus cannot be carried out using standard projection printing systems.
As an alternative, an alternating phase-shift mask may be used in which the light is transmitted only by the clear regions on the mask and in which adjacent clear regions have respective phase shifts of 0 and 180 degrees. The light diffracted into the line regions between the clear regions interfere destructively to improve the image contrast as well as the resolution and depth of focus. However, the active area, bit line and word line masks have patterns that include at lease one array area formed of equal lines and spaces as well as one or more support areas formed of various other patterns. At the ends of the array region, for example, the clear spaces between the solid lines merge so that if an alternating phase-shift mask is used, a space with a 0 degree phase change would intersect with a space having a 180 degree phase change.
To prevent the intersection of two spaces having different phases, one or more dark regions must be included at the ends of the lines to separate the two types of clear areas. However, the patterns printed using such a mask have lines that are shorted to each other and thus would require a second exposure using another mask pattern, typically a standard chrome-on-glass mask pattern, to remove the segments creating the shorts. Additionally, the layout in the support region may also require a double exposure. The double exposure required to incorporate an alternating phase-shift mask pattern requires an additional mask-to-mask alignment step that increases the possibility of introducing alignment errors as well as increasing fabrication costs and personnel expense. The added mask level also increases the possibility of introducing defects. As a result, the use of alternating phase-shift masks are limited to mask levels having only array type patterns that require only a single exposure or to mask levels in which the added cost of a double exposure step cannot be avoided.
It is therefore desirable to provide an alternating phase shift mask pattern that may be used to print a given mask level using only a single exposure.